Frankfurt Airport

Yes, you will do both immigration and later security prior to the departure gate. Could be tight but the Germans are pretty efficient so just hustle along. Save the restroom break till after security.

Frank is right (unfortunately), but my experience with passport control in FRA is excellent....tops 15 minutes. Security can be crazy, depending on the day and time (I've only come in on T2, so T1 might be better), so I would plan for the worst and do your bathroom break on the plane before the pilot hits the seatbelt sign. Kind of a weird route to get to AMS too.....

BTW, if security is really crazy and you're concerned about making your connection, get in line and have one person go up to the front and plead your case. Be nice, and they might cut you a break and let you in. Hope your incoming flight isn't delayed too much.

All you need to know about your transfer in FRA you can find here. Since you are entering the Schengen zone you will go through passport control (normally should take a few min only). You will have also to pass through security again but there are "fast lanes" and in any case it's not comparable to the (some times long) lines of the main security check. If you travel on a single ticket and your incoming flight should be late, LH will have to put you on the next available flight to AMS.

I don't know what gates (concourses) your flight will be using, but when I left from FRA 1½ yrs ago, we were able to move freely between Concourse A (flights inside Schengen) and Concourse Z (above Concourse A, flights to/from outside Schengen). We came from outside the airport (arrived by S-Bahn) and went through security to get into Concourse A. Once inside Concourse A, we had free access to all of the A gates. Then we went through outgoing passport control up to Concourse Z, from which our flight to Denver left.

Of course, if your luggage is not checked through to AMS, and you have to go outside security to claim it, you would have to go through security to get back in, and if your flights are not using A/Z gates, then you probably have to go through security between concourses. I think the A/Z gates are used mostly by Star Alliance airlines, so if you are not staying with a Star Alliance airline, that might affect if you have to go through security.

Also, as I remember, when we arrived in FRA six years ago, we had to leave Z through passport control and go down to A before we could leave the secure zone.

Thanks everyone. I will keep my running shoes ready and keep moving😉